On Sun, Aug 30, 2009 at 06:44:04PM +0400, Michael Tokarev wrote: >> If you lose power with the write caches enabled on that same 5 drive >> RAID set, you could lose as much as 5 * 32MB of freshly written data on >> a power loss (16-32MB write caches are common on s-ata disks these >> days). > > This is fundamentally wrong. Many filesystems today use either barriers > or flushes (if barriers are not supported), and the times when disk drives > were lying to the OS that the cache got flushed are long gone. While most common filesystem do have barrier support it is: - not actually enabled for the two most common filesystems - the support for write barriers an cache flushing tends to be buggy all over our software stack, >> For MD5 (and MD6), you really must run with the write cache disabled >> until we get barriers to work for those configurations. > > I highly doubt barriers will ever be supported on anything but simple > raid1, because it's impossible to guarantee ordering across multiple > drives. Well, it *is* possible to have write barriers with journalled > (and/or with battery-backed-cache) raid[456]. > > Note that even if raid[456] does not support barriers, write cache > flushes still works. All currently working barrier implementations on Linux are built upon queue drains and cache flushes, plus sometimes setting the FUA bit. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
